At present, the transparent electrical conductive film is produced mainly by the sputtering method. There are various modes for the sputtering method, for example, a method of forming a film by allowing inert gas ions, which are generated by direct current or high-frequency discharge, to be accelerated to hit the surface of a target in vacuum so as to strike out atoms constituting the target from the surface for deposition on the substrate surface.
The sputtering method is excellent in that an electrical conductive film having a low surface electric resistance can be formed even if it has a large area to some extent. However, it has a disadvantage that the apparatus is large, and the film forming speed is slow. If the conductive film is to have a still larger area from now on, the apparatus will be further enlarged. This raises a technical problem such that the controlling precision must be heightened and, from another point of view, raises a problem of increase in the production cost. Further, although the number of targets is increased to raise the speed in order to compensate for the slowness of the film forming speed, this also is a factor that enlarges the apparatus, thereby raising a problem.
An attempt is made to produce the transparent conductive film by the application method. In a conventional application method, a conductive paint having conductive fine particles dispersed in a binder solution is applied onto a substrate, dried, and hardened to form the conductive film. The application method has advantages in that a conductive film having a large area can be easily formed, that the apparatus is simple and has a high productivity, and that the conductive film can be produced at a lower cost than by the sputtering method. In the application method, an electric path is formed by contact of the conductive fine particles with each other, whereby the electrical conductivity is exhibited. However, the conductive film produced by the conventional application method has an insufficient contact, and the obtained conductive film has a high electric resistance value (i.e. is inferior in conductivity), thereby limiting its usage.
As an application method using no binder resin, for example, Japanese Laid-open Patent Publication No. 8-199096(1996) discloses a method in which a conductive film forming paint comprising tin-doped indium oxide (ITO) powders, a solvent, a coupling agent and an organic or inorganic acid salt of metal, and not containing a binder is applied onto a glass plate and calcined at a temperature of 300° C. or higher. In this method, since the binder is not used, the conductive film has a low electric resistance value.
Also, a process is known in which a film is formed by application using the sol-gel method. An application method using the sol-gel method is suited for forming a film of large area.
By any of the above-mentioned application methods, in the case that the support is one having flexibility such as a film, a conductive film having a large area can be easily formed, however, in the case that the support is one having poor flexibility such as a plate material, the application is difficult as compared with the case of the flexible support, and particularly it is difficult to control a film thickness for uniformity.
Japanese Laid-open Patent Publication No. 6-103839(1994) discloses a method for manufacturing a transparent conductive substrate by transferring.
Also, Japanese Laid-open Patent Publication No. 2000-207959(2000) discloses a method in which a coating liquid for forming a conductive film containing an organic indium compound, expressed by structural formula In(OH)(OCOR)2 (here, R is alkyl groups that may be a branch or straight chain), and an organic tin compound dissolved in a solvent is applied onto a base material such as glass; and subsequently heat treated to decompose the organic component of the coating liquid by heat thus forming a transparent conductive film. In this method, a conductive film having a low specific resistance can be obtained. In this method, however, when it is tried to obtain a thick conductive film, a crack is produced at the time of heat treatment. Thus, a conductive film having a low surface electric resistance cannot be obtained.
Further, Japanese Laid-open Patent Publication No. 2001-2954(2001) discloses a method in which a coating liquid for forming conductive film containing a formic acid indium compound, an organic tin compound and an organic amine dissolved in a solvent is applied onto a base material such as glass; and subsequently heat treated to decompose the organic component of the coating liquid by heat thus forming a transparent conductive film.